This brochure relates the results of a study made by Grawitz of the growth of pieces of the heart-valves in culture-mediums. The conclusions which he reaches are confirmatory of views which he expressed several years ago, and emphasize the part played by the fixed tissues in processes of growth, repair and inflammation. The development of small, round wandering cells in hanging-drop cultures, the transformation of one type of tissue into another, and the bearing of these and other experimental observations on problems of biology and pathology are clearly stated, and the understanding of the subject made easier, by several excellent illustrations.